Cardiovascular disease is the most frequent cause of death in the United States. Intravascular brachytherapy (IVBT) is an emerging treatment to prevent coronary restenosis after angioplasty. The goal of this project is to develop an accurate three-dimensional (3-D) dosimetry system for IVBT, employing direct Monte Carlo simulation in the patient. This goal is motivated by the hypothesis that the optimal dosimetric window can only be determined with accurate 3-D dosimetry. This hypothesis is driven by the need for evaluating different methods of IVBT treatments and the present inability to provide such evaluation with accurate dose distribution due to the geometric settings of IVBT. it is also driven by the technical advantages of using Monte Carlo simulation to calculate dose in such intricate settings. Many clinical trials are currently underway to determine the efficacy of IVBT. Accurate 3-D dosimetry is crucial for analyzing data from these trails and for determining the optimal dosimetric window for the effectiveness of IVBT. Accurate dosimetry is also needed to improve treatment design, and to provide a better understanding for treatment failure. Currently, the uncertainty in 3-D dosimetry of IVBT is generally high due to the complexity of geometry and the limitation of the conventional dosimetric methods. There is no comprehensive dose determination package or a treatment planning system available for IVBT. Monte Carlo techniques have been shown to model dose deposition accurately even in complicated geometry. Until now, the clinical use of Monte Carlo techniques has been considered prohibitively time consuming but the small region of interest for IVBT and recent developments in computing hardware have eliminated this concern. This project has two specific aims: l). To develop a highly versatile, efficient, and accurate Monte Carlo dosimetry system to calculate 3-D dose distributions in patients for IVBT using intravascular ultrasound images and angiography. 2). To apply the newly developed system to resolve various dosimetric issues, such as (i) source centering, (ii) the impacts in the presence of metallic stents, calcified plagues, contrast media and guide wires, and (iii) dosimetric comparison of various IVBT modalities, and to re- evaluate the dosimetry for the available patient data.